The present invention relates generally to surface cleaning and, more particularly, to a method and apparatus for megasonic cleaning of a semiconductor substrate following fabrication processes.
Megasonic cleaning is widely used in semiconductor manufacturing operations and can be employed in a batch cleaning process or a single wafer cleaning process. For a batch cleaning process, the vibrations of a megasonic transducer creates sonic pressure waves in the liquid of the cleaning tank which contains a batch of semiconductor substrates. A single wafer megasonic cleaning process uses a relatively small transducer above a rotating wafer, wherein the transducer is scanned across the wafer, or in the case of full immersion a single wafer tank system is used. In each case, the main particle removal mechanisms with megasonic cleaning is due to cavitation and acoustic streaming. Cavitation is the rapid forming and collapsing of microscopic bubbles in a liquid medium under the action of sonic agitation. Upon collapse, the bubbles release energy which assists in particle removal through breaking the various adhesion forces which adhere the particle to the substrate. Sonic agitation involves subjecting the liquid to acoustic energy waves. Under megasonic rinsing, these acoustic waves occur at frequencies between 0.4 and 1.5 Megahertz (MHz), inclusive.
FIG. 1A is a schematic diagram of a batch megasonic cleaning system. Tank 100 is filled with a cleaning solution. Wafer holder 102 includes a batch of wafers to be cleaned. Transducer 104 creates pressure waves through sonic energy with frequencies near 1 Megahertz. These pressure waves in concert with the appropriate chemistry to control particle re-adhesion provide the cleaning action. Because of the long cleaning time required for batch cleaning systems, as well as chemical usage, efforts have been focused on single wafer cleaning systems in order to decrease chemical usage, increase wafer-to-wafer control, and decrease defects in accordance with the International Technology Roadmap for Semiconductors (ITRS) requirements. Batch systems suffer from another disadvantage in that the delivery of megasonic energy to the multiple wafers in the tank is non-uniform and can result in ‘hot spots’ due to constructive interference, or ‘cold spots’ due to destructive interference, each being caused by reflection of the megasonic waves from both the multiple wafers and from the megasonic tank. Therefore, a higher megasonic energy as well as multiple transducer arrays must be applied in order to reach all regions of the wafers in wafer holder 102. FIG. 1B is a schematic diagram of a single wafer cleaning tank. Here, tank 106 is filled with a cleaning solution. Wafer 108 is submerged in the cleaning solution of tank 106. Transducer 110 supplies the energy to clean the wafer. Furthermore, particles remain inside the tank requiring that the cleaning fluid be replaced or recirculated and filtered regularly.
FIG. 1C is a schematic diagram of nozzle-type megasonic cleaning configuration for a single wafer. Nozzle 112 provides fluid stream 114 through which the megasonic energy is coupled. Transducer 116, which is connected to power supply 118, provides the megasonic energy through the fluid stream 114 to the substrate as the fluid stream flows through the nozzle. Megasonic energy supplied through fluid stream 114 provides the cleaning mechanism to clean wafer 120. One shortcoming of the nozzle cleaning configuration includes requiring a high flow rate of the fluid stream to maintain contact between transducer 116 and wafer 120. The fluid stream generated through nozzle 112 covers a small area, therefore, a fairly high megasonic energy is needed to clean the wafer, which in turn, may cause damage to the surface of the wafer. The high energy required also necessitates cooling of the transducer, which is another reason for the high flow rates required. This makes cleaning using a cleaning chemistry other than deionized water impractical, due to cost and effluent handling requirements. Banding may also occur because of the difficulty to provide full cleaning coverage of the entire wafer surface with the small coverage area of the fluid stream.
Additionally, the cleaning chemistries for single wafer cleaning processes are highly reactive and often require application at elevated temperatures to provide effective cleaning at low cleaning times, particularly for post etch cleaning applications. Each of the single wafer cleaning configurations described above use batch heating systems with chemical re-circulation, or in the case of a nozzle-type transducer, batch heating with heated delivery lines so that the temperature is maintained for the cleaning chemistry to optimally clean the wafer surface.
In view of the foregoing, there is a need for a method and apparatus to provide a single wafer megasonic cleaning configuration that is capable of heating the cleaning chemistry at the point of contact with the wafer which simplifies heating of the chemistry, and improves process control of the heated chemistry.